Press Release

Screwdriving Technology

(Press Release, September 2012)

Alternative materials used in lightweight construction require different joining techniques

Lightweight automotive bodies take priority

The DEPRAG screwdriving function module sets flowform screws even in confined spaces

Lightweight construction takes priority! The energy-efficient car of the future has an extremely light bodywork structure made from various materials. Conventional assembly is being replaced by alternative technologies. One of these joining techniques is that of flowform fastening. Technical journalist Trixy Schmidt talked to Jürgen Hierold, Sales Manager at screwdriving specialists DEPRAG SCHULZ GMBH u. CO. KG, based in Amberg, Germany, to discover more about the advantages of this joining technique for the lightweight structures now used in the production of vehicle bodies.

Trixy Schmidt: Lightweight vehicles are coming up in the fast lane because it is only such vehicles that meet the demands imposed by government policy and by consumers as regards energy consumption and CO2 emissions. What does that mean for car body construction?

Jürgen Hierold: Today's car body structures combine materials such as steel, aluminium and magnesium alloys. So traditional assembly methods, such as the conventional resistance welding method, are being replaced by alternative joining techniques. Rivets are a possibility if the fastening point is accessible on both sides. If the design prevents this method from being used, screw fastenings made with flowform screws come to the fore as the solution to the problem. Flowform screws create a very high-strength fastening and form a process-reliable joint for different types of sheet metal.

Trixy Schmidt: In flowform fastening technology special screws - available on the market from several respected suppliers - are set in a six step process. But how does the technology actually work?

Jürgen Hierold: The flowform screw is positioned onto the aluminium or steel sheet, which has not been pre-drilled, and the screwdriving process begins. The screw - applied with a high down-force and also rotating at a very high speed - develops a high level of frictional heat, the material begins to flow and a kind of crater forms. In the next step the tip of the screw penetrates the material compound, thus forming a through-funnel. Enough material forms through this funnel shape to allow several thread turns to be applied. This produces a machine thread which can also accept a completely “normal” screw in the event that a repair is required later on. Once the thread has been formed, the screw is tightened to preliminary torque values for the head setting, and the fastening is then tightened to the pre-configured torque and angle parameters.

Trixy Schmidt: So no additional fastening elements such as nuts or bolts are required?

Jürgen Hierold: That's right. Additional fastening elements are not necessary. With this flowform fastening procedure, the nut thread is formed directly inside the through-funnel. And that can only be an advantage. And the fact that all the parameters necessary for process-reliable screw fastening are also available for flowform fastening is crucial. And there is absolutely no need to pre-drill or punch the component. This provides a tremendous rationalisation effect in the manufacture of metal components. The entire screw fastening process has been optimised in such a way that extremely short cycle times can be achieved in assembly. For modern car body construction this translates into a cycle time per screw fastening of less than two seconds!

Trixy Schmidt: Getting back now to the fastening point. Does that mean that the fastening point only needs to be accessible from one side in order to carry out the process? That must be a major advantage in the case of fastening points inside very confined spaces.

Jürgen Hierold: That is precisely the main plus point in favour of flowform fastening. If I could reach both sides of a fastening point, I would probably choose to use rivets. But where I cannot reach both sides, the flowform fastening is the best fastening process to use. The screwdriver only requires access from one side.

Trixy Schmidt: Flowform fasteners have now become an established technique used by car body engineers in the automotive industry. Depending on the design, several hundred screws will be inserted into some larger vehicles. How are the screws fitted in such cases?

Jürgen Hierold: There are special screwdriving systems for this type of fastening technology. As a general rule robots are used to assemble the car bodywork. The robot moves the screwdriving system - which is the “end-of-arm tooling” - to each individual screw position. The screwdriver unit then places screw after screw in a totally reliable process.

Trixy Schmidt: Screwdriving specialist DEPRAG has recently introduced a flowform screwdriving system (FLS) onto the market. As sales manager you are very well aware of what customers want. What features of the new DEPRAG FLS unit would you highlight in particular?

Jürgen Hierold: Our established product lines such as the MINIMAT-EC and MICROMAT-EC stationary screwdrivers, DEPRAG measurement and feeder technology as well as our screwdriving stations and assembly systems have been firm fixtures on the market over several decades. This well-established technology forms the basis of our development work for the new flowform screwdriving system. One of the traditional requirements made by our customers in car body construction is the capacity to perform screw fastening operations in confined spaces. Our designers have addressed this issue in particular.

Trixy Schmidt: Does that mean that the DEPRAG FLS system is particularly suitable for fiddly screwdriving tasks located close to obstacles?

Jürgen Hierold: An outstanding feature of our flowform screwdriving system is its configuration for dealing with fastening points in tight spaces. By using a 1:1 ratio offset gear to offset the drive motor and screwdriving axis to one side the screwdriving system can address screw positions located as little as 14 millimetres away from an obstacle. That is unique.

Trixy Schmidt: The screws are always fed fully automatically into processes of this type. Would you say that the long distances they have to travel in a robot application are an issue?

Jürgen Hierold: Yes, you could say that. But the solution to this problem can be found in our latest development. There are always two flowform screws being fed into the screwdriver system at any one time. While the first is being fastened, the next one is fed through and ready in the waiting position behind the nosepiece. This reduces the time taken to refill the system and it can only contribute to improved cycle times. Moreover the size and weight of the screwdriving systems are optimally adapted to robot applications. Moreover, all the contacts are incorporated into the screwdriver unit in such a way that the unit can be simply and easily connected with the robot. A further highlight is that, with its bayonet fitting, the screw positioner - known as the mouthpiece - can be replaced very quickly without the need for additional tools. When it is necessary to change to a different size of screw for example, or if rapid troubleshooting has to be guaranteed, this is a huge advantage.

Trixy Schmidt: Configuration of the various process steps for flowform fastenings is particularly crucial. While during the heating and flow phases, the system works at maximum down-force and a high revolution rate, when it comes to the thread-forming process, the speed and down-force reduce. The speed and down-force reduce once again for the steps involving preliminary tightening and final torque. So the DEPRAG FLS screwdriver unit has to be very flexible doesn't it?

Jürgen Hierold: Yes, you need an extremely flexible screwdriving system to execute flowform fastenings. For example in car bodies hundreds of screws have to be positioned and, because of the various materials, shapes and sheet thicknesses used, these need to be tightened to different torque parameters. A crucial quality characteristic here is that of achieving the specified torque and rotation angle values. If the conditions at the screw-in position change - for example a different materials pairing, sheet thickness or the screw geometry - then the thread-forming torques usually alter too. To be able to execute every screw fastening with a single screwdriving system, in some circumstances it is not enough to check the traditional torque and rotation angle parameters.

Trixy Schmidt: If the traditional tightening procedure is not enough, what can be done?

Jürgen Hierold: Here DEPRAG offers its own screwdriving processes which, for example calculate the switch-off torque individually each time, based on an analysis of the tapping torque during the screw fastening process (friction coefficient procedure). It is derived by adding a previously defined torque value to the determined tapping torque. So the screw fastening parameters are automatically adjusted to the varying circumstances at each screw-in position.

Trixy Schmidt: How does the system control the various process steps with their differing tightening parameters?

Jürgen Hierold: It uses a modern DEPRAG sequential control system. The process control software (AST) used to control these delicate steps is based on the proven standards used in the sensor-controlled DEPRAG EC-Servo screwdriver. This software is well-established and trusted by its users. It provides a user-friendly interface making it easy to configure the complex flowform fastening process. An additional component is the high-quality DEPRAG PROCESS UNIT (DPU) machine control system with its user-friendly standard software.

Trixy Schmidt: A digital position sensor monitors each step of the complicated joining process. As the fastener approaches and is positioned to the component, the sensor system located in the mouthpiece and in the DEPRAG FLS unit stroke compares the actual status against a configured reference value. What happens if the values do not match?

Jürgen Hierold: If the part's geometry does not match the reference value, perhaps because the metal part has flexed, the screwdriver process and the torque parameters selected automatically adjust to the changed shape of the metal sheet. Only once this has occurred is an enable signal emitted so that the screwdriving process can start. Finally, the last step is to check the condition of the joining process. The parameters are individually analysed, compared against the reference values and, if they are within the tolerance range, confirmed with an “OK” message. This method ensures continuous quality control.

Trixy Schmidt: The motor for the new FLS screwdriving system has to produce high pressing forces and speeds for the first phase during which the crater and the through-funnel are formed. Which drive motor did the DEPRAG designers decide on?

Jürgen Hierold: For the spindle drive motor, the design engineers used the tried and tested EC motor from the MINIMAT and MICROMAT range with a rotation speed of 6000 rpm and torque of 15 Nm. An air cylinder provides the forward feed for the screwdriver's connection with the flowform fastener. A proportional valve controls the air cylinder. Likewise, an air cylinder with a proportional valve provides the high contact pressure at the start of the screw-in procedure (max. 3000 N). The outstanding interplay between the forces is what makes this screwdriving system so efficient and flexible throughout.

Trixy Schmidt: Modern lightweight structures used in the automotive industry are just one example of how flowform screwdriving can be deployed. Where else is it used?

Jürgen Hierold: Basically anywhere where sheet metals need to be fitted together. The advantages of flowform screw fastening are most obvious where the fastening position is accessible from one side only or where it is necessary to achieve rationalisation effects by eliminating the need for through-holes. Apart from car bodies, a typical application can be found in the assembly of “white goods” such as fridges and dishwashers.

Trixy Schmidt: Many thanks for this interesting insight into the innovative technology that is flowform fastening.

In matters of automated assembly, DEPRAG SCHULZ GMBH u. CO. KG is a sought-after partner with a high level of expertise in the field of high-quality EC servo screwdrivers and EC screwdrivers, sophisticated technology, top class feeders, compressed air motors and compressed air tools. As a full service supplier DEPRAG is represented in over 50 countries, employing over 600 people.

Press Contact:

Dagmar Dübbelde
Carl-Schulz-Platz 1
D-92224 Amberg
Tel: 09621 371-343
Fax: 09621 371-199

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